When undesirable thermal excursions occur during the service of products/components, the useful life of the product is reduced. Proper retirement of components or withdrawal of products that experience undesirable thermal excursions prevents failures that could result in loss of life or significant financial loss. For materials such as foods, drugs, electronic components, munitions, and other perishable goods, detection of such excursions allows thermally degraded goods to be identified and removed from use thereby avoiding malfunctions, customer dissatisfaction, and potential lawsuits.
Most products and materials are restricted in allowable thermal excursions. For example, in aircraft, spacecraft and other industrial products, components are designed for use within a certain temperature range. Deviations from the designed allowable temperature range may alter the material, causing degradation in properties leading to increased risk of component failure and reduced lifetime. Knowledge of the thermal history helps in proper retirement withdrawal, in learning causes of undesirable thermal history which might be avoided in future, and in developing a better understanding of failures which might help develop better materials.
While temperature overshoots can hurt the life of any material, excursions into certain critical temperature ranges below the operating temperature are also known to affect the life in certain materials. Such materials include SiC/BN/SiC composites which suffer oxidation-induced loss in properties and intermetallic compounds including MoSi2 which suffers from “pesting”, a phenomenon of enhanced oxidation in the intermediate temperature regime of 600-1000° C.
In other products, thermal fluctuations beyond the allowed range will degrade the product with no obvious indications. For example, materials such as foods, drugs, electronic components, munitions, and other perishable goods can spoil or degrade due to inadvertent thermal exposure during production, shipping, handling, or on the shelf at the retailer. As a consequence of spoilage and degradation, the materials are no longer usable or marketable resulting in losses to producers and retailers and in the inability to use the materials by the consumer.
Attempts have been made to obtain more information about the time of exposure at a given temperature. For example, U.S. Pat. No. 5,975,758 to Yokota et al. discloses a method and sensor for detecting the thermal history of a specimen. The sensor is made up of a pair of electrodes, a diffusion layer of insulating material disposed between the electrodes, and an electrically conductive metal. The electrodes are electrically isolated from each other in the beginning. As the sensor experiences thermal exposure, the electrically conductive metal diffuses into the diffusion layer, thereby changing the resistance of the diffusion layer. When in use, the sensor is placed in or near the atmosphere to which the specimen is exposed. In response to temperature, the electrically conductive metal diffuses into the diffusion layer to such an extent that the electrical resistance across the electrodes extremely decreases after a certain period of time. Thus, it is possible to detect the deterioration of life of the specimen by monitoring the change in electrical resistance of the sensor.
While some inventions have been made to determine thermal history, there exists a need for a simple device capable of monitoring the thermal history of such materials to assure quality and prevent spoilage and degradation. In particular, the device should be capable of identifying detrimental thermal exposures and identifying when such exposures occurred.